In environmental, physical, chemical and microbiological studies, filtration is often used to concentrate small particles (e.g. microbial cells) suspended in minute concentration in fluids or gases. In general, it may be necessary to separate and concentrate small particles according to the particle dimension. Further, it is desirable to rapidly and completely capture these small particles on a filter.
For example, filtration-based particle concentration techniques have been used to isolate and recover waterborne pathogens into small volume for downstream analysis. Such isolated/recovered microorganisms are more readily accessible to different detection methods using fluorescent probes. For example, Cryptosporidium parvum oocyst is a parasite commonly found in surface waters such as lakes and rivers, especially when the water is in contact with animal wastes and sewage. The highly infectious nature of C. parvum oocyst and the lack of effective medication until now urge a reliable routine test to monitor C. parvum oocyst contamination in the drinking water supply system [1]. Available bio-sensors only detect microorganisms which are directly in contact with the sensitive region; hence, detection of low concentration bacteria in a large volume is hard because there is low possibility for bacteria to interact with the sensitive zone of biosensor [2]. Consequently, a reliable method to concentrate C. parvum oocysts present in large volume of drinking water into smaller volume is crucial for accurate detection and quantification of C. parvum oocysts from drinking water. Filtration-based concentration techniques have been widely used for capturing and recovering C. parvum oocysts into small volume for downstream analysis [3].
Negative features of commercial filters like rough surface, tortuous pore path, low pore density and high coefficient of variation [4] (CV>20%) are the major factors which compromise their efficiency and throughput during microfiltration. The best homogeneity in terms of pore size distribution and pore shape in commercial membranes is possessed by the track-etched polymeric membranes (normally polycarbonate) by cylindrical pores, but the irregular array of pores on the surface, low porosity and also their angle with the surface limits the strength, flow rate and reliability of them in repeated process-scale. Therefore, these filters are normally employed in single-use laboratory analysis [3,5].
From the last decades, different methods have been proposed to produce membranes with micro/nano cylindrical pores, high porosity and good mechanical strength. For instance, optical lithography has been employed to fabricate silicon nitride membranes with a pore size of around 0.1 μm. Identical and uniform pore diameter and smooth surface allow the filtration membrane to have low transmembrane pressures and large flux, but the restriction in membrane material (limited to nitride) and small thickness of the silicon nitride film (<1 μm), which allows only low working pressure (<2 bar) limits the application of this type of filtration membrane [6,7]. Fabrication of polymeric through-hole membrane using high aspect ratio metal moulds with a hole-diameter down to hundreds of nanometers is also investigated by Yanagishita et al. [8]. A major impediment in fabrication of membrane with this method is the demoulding of cured membrane, because peeling off of the membrane from the mould often results in membrane damage and failure. Furthermore, the choices of pore size and pore density are restricted due to the use of alumina templates. In another study, polymeric sieves have been fabricated using aperture array lithography [9]. In this method, aperture array lithography and reactive ion etching (RIE) techniques have been combined to obtain polymeric membranes with a homogeneous pore diameter on the scale of hundreds of nanometers, but it is a costly process which is not appropriate for large-scale production. More recently, polymeric membranes were obtained by phase separation micromolding [10]. In spite of capability of this method in employing a variety of polymers for filter design, some major problems such as enlargement of pore size during the shrinkage stage, fragility of the mould and folding (or failure) of membrane during the release stage are associated with this method.
In all the aforementioned methods, it is difficult or perhaps even impossible to make a membrane with small features (i.e. down to hundreds of nanometer) without compromising the membrane strength. As a “rule of thumb,” for instance, the diameter or the largest transverse dimension of the pore cannot be smaller than about half of the membrane thickness [7]. Hence, in order to obtain membrane with pore size of around 1 μm or less, the membrane thickness would be smaller than 2 μm which is so fragile that it cannot be used for microfiltration applications.
It is therefore desirable to further improve on filters and fabrication methods of filters. Technically, fabrication of small perforations (e.g 0.1-5 μm) inside a thick photoresist film (e.g. SU-8) is difficult due to the tapering effect, which normally happens during UV exposure (i.e. usually, the top layer is overexposed and tends to be wider than the bottom layer which is relatively underexposed, resulting in variation in the lateral dimensions).